This invention relates to a process of forming a membrane electrode assembly that comprises a composite membrane and is suitable for use in electrochemical devices, including proton exchange membrane fuel cells, electrolyzers, chlor-alkali separation membranes, sensors, and the like.
Electrochemical devices, including proton exchange membrane fuel cells, electrolyzers, chlor-alkali separation membranes, and the like, have been constructed from membrane electrode assemblies (MEAs). Such MEAs comprise one or more electrode portions, which include a catalytic electrode material such as Pt or Pd, in contact with an ion conductive membrane. Ion conductive membranes (ICMs) are used in electrochemical cells as solid electrolytes. In a typical electrochemical cell, an ICM is in contact with cathode and anode electrodes, and transports ions such as protons that are formed at the anode to the cathode, allowing a current of electrons to flow in an external circuit connecting the electrodes.
MEAs are used in hydrogen/oxygen fuel cells. A typical MEA for use in a hydrogen/oxygen fuel cell might comprise a first Pt electrode portion, an ICM comprising a proton-exchange electrolyte, and a second Pt electrode portion. Such an MEA can be used to generate electricity by oxidation of hydrogen gas, as illustrated in the following reactions: 
In a typical hydrogen/oxygen fuel cell, the ions to be conducted by the membrane are protons. Importantly, ICMs do not conduct electrons/electricity, since this would render the fuel cell useless, and they must be essentially impermeable to fuel gasses, such as hydrogen and oxygen. Any leakage of the gasses employed in the reaction across the MEA results in waste of the reactants and inefficiency of the cell. For that reason, the ion exchange membrane must have low or no permeability to the gasses employed in the reaction.
ICMs also find use in chlor-alkali cells wherein brine mixtures are separated to form chlorine gas and sodium hydroxide. The membrane selectively transports sodium ions while rejecting chloride ions. ICMs also can be useful for applications such as diffusion dialysis, electrodialysis, and pervaporization and vapor permeation separations. While most ICMs transport cations or protons, membranes that are transportive to anions such as OHxe2x88x92 are known and commercially available.
Commercially-available ICMs are not entirely satisfactory in meeting the performance demands of fuel cells. For example, Nafion(trademark) membranes (DuPont Chemicals, Inc., Wilmington, Del.) which are perfluorocarbon materials having a SO3xe2x88x92 anion, are inherently weak. Nafion(trademark) membranes are not generally available at thicknesses of less than 50 xcexcm. One reason is that Nafion(trademark) membranes that thin would require reinforcement, thus defeating the purpose of a thin membrane by increasing the overall thickness as well as increasing the electrical resistance of the membrane. While Nafion(trademark) membranes with lower equivalent weight can be used to obtain lower electrical resistance, lower equivalent weight membranes are structurally weaker and still would not obviate the need for reinforcement.
One means of constructing a reinforced membrane is to imbibe or infuse an ion-conductive material into a porous inert reinforcing membrane to make a composite membrane. For example, Gore-Select(trademark) membranes (W. L. Gore and Associates, Inc., Elkton, Md.) comprise a poly(tetrafluoroethylene) (PTFE) membrane having an ion-conductive or ion exchange liquid impregnated therein. U.S. Pat. No. 5,547,551 describes a PTFE membrane fully impregnated with Nafion(trademark) solution for use in fuel cells. Other inert membranes have been mentioned, such as polyolefins and poly(vinylidene fluoride), as suitable carriers for ion-conducting electrolytes.
Composite proton exchange membranes, comprising electrolytes immobilized in porous webs, have been shown to offer superior properties over single component membranes when used in fuel cells. The composite membranes can be made thinner and stronger while giving equivalent conductivity with less electrolyte, and have more dimensional stability even after becoming saturated with water. However, because the membranes employed are initially porous, the gas permeability of the resulting membrane depends in part on the degree to which the membrane is filled by the electrolyte.
These composite membranes are used in fuel cell MEAs that use conventional catalyst electrodes in the form of applied dispersions of either Pt fines or carbon supported Pt catalysts. These conventional catalysts are applied as a coating of ink or paste to either the composite membrane or to an electrode backing layer placed adjacent to the membrane. The ink or paste typically contains electrolyte in the form of an ionomer.
Various structures and means have been used to apply or otherwise bring a catalyst in contact with an electrolyte to form electrodes, e.g., cathodes and anodes. These xe2x80x9cmembrane electrode assembliesxe2x80x9d (MEAs) can include: (a) porous metal films or planar distributions of metal particles or carbon supported catalyst powders deposited on the surface of the ICM; (b) metal grids or meshes deposited on or imbedded in the ICM; or (c) catalytically active nanostructured composite elements embedded in the surface of the ICM.
Nanostructured composite articles have been disclosed. See, for example, U.S. Pat. Nos. 4,812,352, 5,039,561, 5,176,786, 5,336,558, 5,338,430, and 5,238,729. U.S. Pat. No. 5,338,430 discloses that nanostructured electrodes embedded in solid polymer electrolyte offer superior properties over conventional electrodes employing metal fines or carbon supported metal catalysts, including: protection of the embedded electrode material, more efficient use of the electrode material, and enhanced catalytic activity.
Briefly, this invention provides a method of making a membrane electrode assembly that comprises a composite membrane, which includes both a porous membrane and an ion conducting electrolyte, by partially filling a porous membrane with an ion conducting electrolyte to form a partially filled membrane and then compressing together the partially filled membrane and electrode particles so as to remove void volume from the partially filled membrane and embed the electrode particles in the partially filled membrane. The membrane electrode assembly of this invention is suitable for use in electrochemical devices, including proton exchange membrane fuel cells, electrolyzers, chlor-alkali separation membranes, sensors and the like.
In another aspect, the present invention provides a composite membrane including a polymerization product comprising one or more monomers having the formula CH2xe2x95x90CHxe2x80x94Arxe2x80x94SO2xe2x80x94SO2(C1+nF3+2n), wherein n is 0-11, preferably 0-3, and most preferably 0, and wherein Ar is any substituted or unsubstituted aryl group, preferably of molecular weight less than 400 and most preferably a divalent phenyl group.
In a further aspect, the invention provides a fuel cell assembly comprising at least one membrane electrode assembly disclosed above.
In yet another aspect, the invention provides an electrochemical device comprising at least one MEA disclosed above.
In the method of the present invention, a porous membrane is partially filled with an ion conducting electrolyte to form a partially filled membrane. The partially filled membrane is then pressed with electrode particles so as to embed the electrode particles in the partially filled membrane. It was found that this pressing step also removed void volume remaining after the filling step, and therefore resulted in a thinner and less porous composite membrane than previously contemplated. In a preferred embodiment, the present invention provides a method for forming a membrane electrode assembly that comprises embedded electrode particles, which may be nanostructured catalyst particles, together with a composite membrane.
Furthermore, under certain circumstances it was observed that, not only was the void space of the porous membrane filled, but the porous structure itself was obliterated. Under a scanning electron microscope the resulting membrane appeared uniform, even at a magnification of 10,000xc3x97. Thus, in another preferred embodiment, the present invention provides a method for forming a membrane electrode assembly that comprises a composite membrane which has acquired a uniform, undifferentiated structure, that is, wherein the porous structure of the initially porous membrane is obliterated.
In addition, resulting MEA""s were shown to function well in electrochemical cells.
In this application:
xe2x80x9ccomposite membranexe2x80x9d means a membrane composed of more than one material and including both a porous membrane material and an ion conducting electrolyte material;
xe2x80x9cmembrane electrode assemblyxe2x80x9d means a structure comprising a membrane that includes an electrolyte and at least one but preferably two or more electrodes adjoining the membrane;
xe2x80x9csubstitutedxe2x80x9d means, for a chemical species, having a conventional substituent that does not interfere with the desired product;
xe2x80x9cnanostructured elementxe2x80x9d means an acicular, discrete, sub-microscopic structure comprising an electrically conductive material on at least a portion of its surface;
xe2x80x9cacicularxe2x80x9d means having a ratio of length to average cross-sectional width of greater than or equal to 3;
xe2x80x9cdiscretexe2x80x9d refers to distinct elements, having a separate identity, but does not preclude elements from being in contact with one another;
xe2x80x9csub-microscopicxe2x80x9d means having at least one dimension smaller than about a micrometer;
xe2x80x9cGurley numberxe2x80x9d means a measure of the resistance to gas flow of a membrane, expressed as the time necessary for a given volume of gas to pass through a standard area of the membrane under standard conditions, as specified in ASTM D726-58, Method A, described further below; and
xe2x80x9cpore sizexe2x80x9d means a measure of size of the largest pore in a membrane as specified in ASTM F-316-80, described further below.
It is an advantage of the present invention to provide a method of making a strong, thin, and more gas impervious membrane electrode for use in membrane electrode assemblies. In particular, it is an advantage of the present invention to provide a method of making a membrane electrode comprising a thinner and more completely filled composite membrane with nanostructured electrodes. In addition, it is an advantage of the present invention to provide a method of making a membrane electrode comprising a thin and non-porous composite membrane lacking any visible porous structure and having nanostructured electrodes.